Carbon nanotubes (CNTs) can have different electronic properties based at least in part on their physical structure. For example, the chirality of a single-walled CNT (SWNT) impacts the conductivity of the tube, such that it may be either metallic or semiconducting. Different types of CNTs can have uses for different applications; for example, photovoltaic and photoelectric devices using semiconducting CNTs are known from US Patent Application Publication No. 2010/0065829A1 (hereinafter Ref. [1]). Efficient and/or proper operation of the CNT-based device may depend on the purity of CNTs used so that, for example, the presence of even a small percentage of metallic CNTs in a batch of semiconducting CNTs used for a particular application may undesirably affect the device's operation.
Bulk methods for creating CNTs result in the production of both semiconducting and metallic CNTs in a ratio of approximately one part metallic for two parts semiconducting. For at least some practical applications of CNTs as noted above, it may be beneficial or even necessary to separate these two types of CNTs so as to isolate the desired electrical type. Further complicating this separation is the fact that CNTs tend to bundle together due to Van der Waals force.
One known method involves the use of density-gradient ultracentrifugation (DGU) which physically separates the metallic and semiconducting CNTs into different density layers. Bundles of CNTs, aggregates, and insoluble material present in the solution sediment out to lower levels (higher density) in the gradient. See M. Arnold et al., “Sorting carbon nanotubes by electronic structure using density differentiation,” Nature Nanotechnology 60, Vol. 1, October 2006 pp. 60-65 (hereinafter Ref. [2]).
Another known method for separating metallic and semiconducting CNTs involves first sonicating the bulk-produced CNTs using sodium dodecyl sulfate (SDS) to obtain some singly-dispersed CNTs along with some bundles of CNTs, ultracentrifuging to sediment out large bundles and impurities, and then using a chromatography column with an agarose gel bead stationary phase to filter out the semiconducting CNTs. The semiconducting CNTs were then eluted from the agarose beads using sodium deoxycholate. See T. Tanaka et al., “Continuous Separation of Metallic and Semiconducting Carbon Nanotubes Using Agarose Gel,” Appl. Phys. Express 2 (2009) 125002 (hereinafter Ref. [3]). Refs. [1], [2], and [3] are hereby incorporated by reference.
These methods of isolating individual CNTs of a particular electronic type are somewhat limited in their ability to obtain highly purified nanotubes. For devices that utilize one electronic type, this purity may be important or may impact efficiency or other performance parameters of the end device.